Recently, water-absorbent resins have been used in the sanitary field, for example, in menstrual articles, diapers and the like, the agricultural and horticultural fields, for example, in water retention agents, soil conditioning agents and the like and industrial fields, for example, in cutoff materials, anti-dewing agents and the like, and they are useful in various applications. Among these, water-absorbent resins are advantageously used in the sanitary field in menstrual articles, diapers and the like.
One kind of these water-absorbent resins is a polymer which is lightly crosslinked, and examples thereof include hydrolysates of starch-acrylonitrile graft copolymer (Japanese Patent Kokoku No. 49-43395), neutralized products of starch-acrylic acid graft copolymer (Japanese Patent Kokai No. 51-125468), specified products of vinyl acetateacrylic acid ester copolymer (Japanese Patent Kokai No. 52-14689), partially neutralized polyacrylic acid products (Japanese Patent Kokai Nos. 62-172006, 57-158209 and 57-21405) and the like.
Usually, the properties desired for water-absorbent resins are a high absorbency, an excellent water-absorption rate, a high gel strength after absorption of water, a shape suitable for application, conformability to a material to be used therewith and the like. Particularly, the desired properties for water-absorbent resins used for sanitary materials are a high absorbency, a high water-absorption rate, a high gel strength after absorption of water, large particle size, less fine powder, a sharp distribution of particle size, integrity with a pulp, a small amount of reversion of absorbed substances the exterior, excellent diffusion of absorbed substances into interior of the absorbents and the like. It can be said that a good water-absorbent material should satisfy these properties as well as be safe and cheap. Thus, hitherto, water-absorbent resins have been mainly produced by solution polymerization or reversed phase suspension polymerization of water-soluble ethylenic unsaturated monomers. Among these, the production of water-absorbent resins by reversed phase suspension polymerization has following disadvantages.
Firstly, a water-absorbent resin obtained by reversed phase suspension polymerization method of a water-soluble ethylenic unsaturated monomer is powder of spherical particles having a sharp distribution of particle size in comparison with that obtained by subjecting the monomer to solution polymerization, followed by grinding. However, a water-absorbent resin having large particle size cannot obtained. Secondly, since a surfactant and/or a polymeric protective colloid are used, they remain on the surface of a product and therefore initial wettability by water is inferior due to water repellency of the surfactant and/or polymeric protective colloid. As means for mitigating this phenomenon and improving initial wettability by water, it has been considered to remove the surfactant and/or polymeric protective colloid from a product by filtering a slurry obtained by reversed phase suspension polymerization and drying. However, purification of the filtrate is very expensive and such a process is far from an economical production process, although initial wettability by water is improved. Thirdly, the polymerization of a water-soluble ethylenic unsaturated monomer is an exothermic reaction and heat is generated within a short period of time. Therefore, in the reversed phase suspension polymerization, an increase in amount of the monomer in a solvent is limited due to removal of heat. Accordingly, improvement of productivity by decreasing amount of the solvent and increasing the amount of the monomer is limited. Fourthly, in the reversed phase suspension polymerization, the surfactant and/or polymeric protective colloid should be used, at least, in an amount sufficient for making a suspension in order to carry out a stable polymerization, and the amount cannot be reduced to less than this required minimum amount.